1. Field of the Invention
This invention relates to a system for controlling the power of a laser used in connection with optical fiber and more particularly to such a system which automatically controls the power output of the laser to ensure maximum dynamic range for various fault conditions and also automatically compensates for changes in laser characteristics arising as a result of changes in temperature and aging.
2. Description of the Prior Art
Providers of telecommunications transmission services are increasingly making use of optical fiber. There already exists today several networks which provide long distance data and/or voice services over such fiber. The fiber is usually buried in the ground. Lasers are used in connection with such services for not only providing the signal(s) to be transmitted on such fibers but also for testing the fibers.
One such testing use is for determining the length and attenuation of a reel of optical fiber. Another such use is in an optical time domain reflectometer (OTDR) for determining if a fault such as a break has occurred on the fiber and the location of the fault. A more detailed discussion of the use of lasers in reflectometers as well as various prior art embodiments therefor may be obtained by referring to U.S. Pat. Nos. 4,606,632; 4,674,872; 4,708,471; 4,732,469 and 4,737,026.
As can well be appreciated buried fiber can accidentally be cut by digging. In some instances the persons doing the digging may not even know that they have cut the fiber. It is important therefore for the telecommunications service provider to be able to easily determine if a fault such as a break has occurred on the fiber and the location of the fault.
At present the service provider is usually first notified of a fault on the fiber and then must send a technician with an OTDR to the office nearest to the fault. The technician connects the OTDR to the fiber through a fiber optic connector and then proceeds to control the power of the laser in the OTDR in an attempt to locate the fault. The time it takes to locate the distance that the fault is from the office and the accuracy with which that measurement is made depends to a great extent on the skill of the technician. It is, however, desirable to have an OTDR which can be directly and permanently connected to optical fiber. It is further desirable that such an OTDR be capable of automatically compensating for changes in laser characteristics. It is also further desirable that when such an OTDR determines that a fault has occurred that it automatically and accurately provides the location of the fault from the office where it is located.
For such an OTDR the location of the fault from the office no longer depends on the skill of the technician. In addition, such an OTDR can be connected to the fiber in an unattended office as a technician is no longer needed to adjust the OTDR. The occurrence of a fault can then be signaled to a remote manned site. A technician can then be sent to the office signaling the fault. All that the technician has to do is read the output of the OTDR to find the location of the fault. The laser power control of the present invention allows for an OTDR which meets the above requirements.